1. Field of the Invention
The present invention relates to a mobile electronic device comprising a sapphire cover plate, particularly a cover plate comprising a modified sapphire having a dielectric constant greater than the dielectric constant of sapphire.
2. Description of the Related Art
There are many types of mobile electronic devices currently available which include a display window assembly that is at least partially transparent. These include, for example, handheld electronic devices such media players, mobile telephones (cell phones), personal data assistants (PDAs), pagers, tablets, and laptop computers and notebooks. The display screen assembly may include multiple component layers, such as, for example, a visual display layer such as a liquid crystal display (LCD), a touch sensitive layer for user input, and at least one outer cover layer used to protect the visual display. Each of these layers are typically laminated or bonded together.
Touch sensitive screen applications are very popular for many mobile electronic devices because they eliminate mechanical push button components that can wear and fail and because they can be incorporated into as more visually appealing form factor for the device. The majority of such touch screens rely on sensing the capacitance within electronic circuits that are mounted in close proximity to or constructed upon the window assembly. These capacitance sensing touch screens have the advantage over other technology that they have no moving parts and do not rely on pressure or flexure to actuate, eliminating wear and extending useful lifetime.
The capacitance sensing electronics in these touch screens are used to locate the position of a probe, such as a user's finger, with respect to the screen. As the probe approaches die electronic circuits, the native capacitance in circuits is altered by the additional capacitance provided by the electric field surrounding the probe. This phenomenon is referred to as projected capacitance.
The capacitance sensing components of the touch screen electronics are distributed along some grid on a horizontal plane. The electronic circuit is designed to locate the x and y coordinates of those components most strongly affected by the electric field of the approaching probe, thus locating the probe position with respect to the touch screen coordinates. These touch screen electronic circuits are designed to maximize the precision with which this coordinates are determined. Higher precision improves the ease of use and the functionality a the user interface provided by the touch screen. The sensitivity of these circuits to the electric field surrounding the probe is directly impacted by the electrical properties of the material used to prepare the touch screen, including its dielectric constant.
In addition, the mechanical properties of the touch screen material are also important. Many of the mobile electronic devices used today are subjected to excessive mechanical and/or chemical damage, particularly from careless handling and/or dropping, from contact of the screen with items such as keys in a user's pocket or purse, or from frequent touch screen usage. For example, the touch screen surface and interfaces smartphones and PDAs can become damaged by abrasions that scratch and pit the physical user interface, and these imperfections can act as stress concentration sites making the screen and/or underlying components more susceptible to fracture in the event of mechanical or other shock. Additionally, or from the use's skin or other debris can coat the surface arid may further facilitate the degradation of the device. Such abrasion and chemical action can cause a reduction in the visual clarity of the underlying electronic display components, thus potentially impeding the use and enjoyment of the device and limiting its lifetime.
Various methods and materials leave been used in order to increase the durability of the display windows of mobile electronic devices. For example, polymeric coatings or layers can be applied to the touch screen surface in order to provide a barrier against degradation. However, such layers can interfere, with the visual clarity of the underlying electronic display as well as interfere with the touch screen sensitivity. Furthermore, as the coating materials are often also soft, they can themselves become easily damaged, requiring periodic replacement or limiting the lifetime of the device.
Another common approach is to use more highly chemically and scratch resistant materials as the outer surface of the display window. For example, touch sensitive screens of some mobile devices may include a layer chemically-strengthened alkali aluminosilicate glass, with potassium on replacing sodium ions for enhanced hardness, such as the material referred to as Gorilla® glass available from Corning. However, even this type of glass can be scratched by many harder materials, including metal keys, sand, and pebbles, and, further, as a glass, is prone to brittle failure or shattering. Sapphire has also been suggested and used as a material for either the outer layer of the display assembly or as a separate protective sheet to be applied over the display window. However, sapphire is relatively expensive, particularly at the currently available thicknesses.
Thus, while materials are available which can enable the display of a mobile electronic device to be relatively resistant to damage, there remains a need in the industry for materials that provide improved mechanical toughness and scratch resistance without reducing the propagation of the electric field, particularly for touch sensitive screen applications.